Tropical Cyclone Rainfall Michael Brennan National Hurricane Center Outline • Tropical Cyclone (TC) rainfall climatology • Factors influencing TC rainfall • TC rainfall forecasting tools • TC rainfall forecasting process Tropical Cyclone Rainfall Climatology Tropical Cyclone Tracks COMET (2011) Global Mean Monthly TC Rainfall During the TC Season and Percent of Total Annual Rainfall Data from TRMM 2A25 Precipitation Radar from 1998-2006 Jiang and Zipser (2010) Contribution to Total Rainfall from TCs • What percentage of average annual rainfall in southern Baja California, Mexico comes from tropical cyclones? 1. 10-20% 2. 20-30% 3. 40-50% 4. 50-60% Khouakhi et al. (2017) J. Climate Contribution to Global Rainfall from TCs (1970-2014 rain gauge study) • Globally, highest TC rainfall totals are in eastern Asia, northeastern Australia, and the southeastern United States • Percentage of annual rainfall contributed by TCs: • 35-50%: NW Australia, SE China, northern Philippines, Baja California • 40-50%: Western coast of Australia, south Indian Ocean islands, East Asia, Mexico Khouakhi et al. (2017) J. Climate Contribution to Global Rainfall from TCs (1970-2014 rain gauge study) Khouakhi et al. (2017) J. Climate Contribution to Global Rainfall from TCs (1970-2014 rain gauge study) • Relative contribution of TCs to extreme rainfall • Gray circles indicate locations at which TCs have no contribution to extreme rainfall Khouakhi et al. (2017) J. Climate Annual TC Rainfall Equator TC Rain . TC rainfall makes up a larger percentage of total rainfall during years when global rainfall is low . Asymmetric - generally 35 000 0 1998 (cm) 1999 (cm)30 000 0 2000 (cm) 2001 (cm) 2002 (cm)25 000 0 2003 (cm) 2004 (cm) 2005 (cm)20 000 0 15 000 0 10 000 0 50 000 0 -40 -20 0 20 40 more TC rainfall in the Northern Hemisphere TC Rain % of Total Rain . TCs produce 10-17% of global rain from 15-25°N . TCs produce 5-10% of global rain from 15-25°S Frank Marks (HRD) Biggest TC Rain Producers By Country/Island Belize 829.8 mm 32.67” Keith (2000) Bermuda 186.7 mm 7.35” October 1939 Hurricane Canada 302.0 mm 11.89” Harvey (1999) Cayman Islands 764.8 mm 31.29” Sanibel Island Hurricane (1944) Costa Rica 920.0 mm 36.22” Cesar (1996) Cuba 2550.0 mm 100.39” Flora (1963) Dominica 422.3 mm 16.63” Jeanne (2004) Dominican Rep. 1001.5 mm 39.43” Flora (1963) El Salvador 406.4 mm 16.00” Adrian (2005) Guadeloupe 508.0 mm 20.00” Marilyn (1995) Guatemala 600.0 mm 23.62” Mitch (1998) Haiti 1447.8 mm 57.00” Flora (1963) Honduras 912.0 mm 35.89” Mitch (1998) Jamaica 3429.0 mm 135.00” November 1909 Hurricane Martinique 680.7 mm 26.80” Dorothy (1970) Mexico 1576.0 mm 62.05” Wilma (2005) Nicaragua 1597.0 mm 62.87” Mitch (1998) Panama 695.0 mm 27.36” Mitch (1998) Puerto Rico 1058.7 mm 41.68” T.D. #19 (1970) St. Lucia 668.0 mm 26.30” Tomas (2010) St. Martin/Maarten 866.6 mm 34.12” Lenny (1999) Venezuela 339.0 mm 13.30” Brett (1993) Original Source: David Roth WPC (2006) Biggest TC Rain Producers By Country/Island Belize 829.8 mm 32.67” Keith (2000) Bermuda 186.7 mm 7.35” October 1939 Hurricane Canada 302.0 mm 11.89” Harvey (1999) Cayman Islands 764.8 mm 31.29” Sanibel Island Hurricane (1944) Costa Rica 920.0 mm 36.22” Cesar (1996) Cuba 2550.0 mm 100.39” Flora (1963) Dominica 422.3 mm 16.63” Jeanne (2004) Dominican Rep. 1001.5 mm 39.43” Flora (1963) El Salvador 406.4 mm 16.00” Adrian (2005) Guadeloupe 508.0 mm 20.00” Marilyn (1995) Guatemala 600.0 mm 23.62” Mitch (1998) Haiti 1447.8 mm 57.00” Flora (1963) Honduras 912.0 mm 35.89” Mitch (1998) Jamaica 3429.0 mm 135.00” November 1909 Hurricane Martinique 680.7 mm 26.80” Dorothy (1970) Mexico 1576.0 mm 62.05” Wilma (2005) Nicaragua 1597.0 mm 62.87” Mitch (1998) Panama 695.0 mm 27.36” Mitch (1998) Puerto Rico 1058.7 mm 41.68” T.D. #19 (1970) St. Lucia 668.0 mm 26.30” Tomas (2010) St. Martin/Maarten 866.6 mm 34.12” Lenny (1999) Venezuela 339.0 mm 13.30” Brett (1993) Original Source: David Roth WPC (2006) Characteristics of TC Precipitation Stratiform and Convective Mechanisms Stratiform Rain ~50% of Total Rain from TC NOAA/HRD - Daily Radar Rainfall Estimate Study Typical warm season 1-day total Hurricane Irene (1999) 1-day total Primarily Stratiform Convective & Convective Hurricane Irene (15 October 1999) Frank Marks (HRD) TC Rainband Complexes: Stratiform Region Didlake and Houze (2013) • Broad stratiform band in left-of-shear half of the storm • Mesoscale ascending outflow & descending inflow driven by latent heating & latent cooling patterns • Increased rainfall along line where descending inflow halts • Descending inflow strengthens the outer core of vortex TC Rainband Complexes: Convective Cells Didlake and Houze (2013) Cell 1 Cell 2 • Weaker, shallower reflectivity core • More intense reflectivity, heavier rain • Weaker updraft • Increased CAPE, more buoyant updraft • Shallower, but stronger inflow layer • Deeper inflow layer • Tangential jet and outflow confined • Tangential jet and outflow extend to lower levels deeper into the troposphere Factors Influencing Tropical Cyclone Rainfall What Factors Influence Rainfall from Tropical Cyclones? • Which of the following is NOT a primary factor in determining rainfall from tropical cyclones? 1. TC Track 2. TC Size 3. TC Intensity 4. Topography What Factors Influence Rainfall from Tropical Cyclones? • Movement – slow forward motion can produce more rain • Storm size – the larger the storm, the greater the area typically receiving rain • Storm track – determines the location of the rain • Diurnal cycle – heaviest rainfall generally near the storm center overnight, outer band rainfall during the day • Topography – enhances rainfall in upslope areas, but decreases rainfall past the spine of the mountains • Moisture – entrainment of dry air can redistribute and/or reduce the amount of precipitation; increased moisture can increase rainfall • Interaction with other meteorological features (troughs, fronts, jets) and extratropical transition can greatly modify rainfall distribution Factors Influencing TC Rainfall Storm Motion Rainfall in Inches • Slow vs. fast moving 1 Inch = 25.4 mm TCs Hurricane • TCs with a turning or Mitch fatalities: looping track vs. Honduras: 5,677 straight mover Nicaragua: 2,863 Guatemala: 258 El Salvador: 239 Vulcan Casita, Nicaragua - debris flows David Roth WPC Tegucigalpa, Honduras river flooding Factors Influencing TC Rainfall Storm Size Determined by distance from center to outermost closed isobar <2 degrees “Very small/ Marco midget” (2008) 2-3 degrees “Small” Ida (2009) 3-6 degrees “Average” Frances (2008) 6-8 degrees “Large” Wilma (2008) >8 degrees “Very large” Sandy (2012) Original Source: Joint Typhoon Warning Center Factors Influencing TC Rainfall Time of Day Alberto, July 4-5, 1994 04/18z 00z 05/06z 12z 18z Factors Influencing TC Rainfall Time of Day Alberto, July 4-5, 1994 04/18z 00z 05/06z 12z 18z Factors Influencing TC Rainfall Terrain Impacts Heaviest rainfall favors mountains perpendicular to the wind Rainfall in Inches 1 Inch = 25.4 mm David Roth WPC Hurricane Georges in Puerto Rico $1.75 billion in damage 28,005 homes destroyed Factors Influencing TC Rainfall Vertical Wind Shear – Northern Hemisphere Shear directed across the storm track leads to more uniform distribution of the rainfall Shear directed parallel to the storm track leads to a distribution of the rainfall asymmetry on the left side of the track Rogers et al, 2003 Factors Influencing TC Rainfall Shear, Mesovortices, and Topography • Downshear region of strong convection associated with Erika (2015) passed directly over Dominica, producing over 500 mm of rainfall • Driven by 500-850 mb shear rather than deep layer shear • Meosvortex on the scale of ~ 100 km developed within Erika’s circulation and persisted over Dominica for 3 hours, likely due to topographic effects, enhancing heavy rainfall Nugent and Rios-Berrios (2018, MWR) Factors Influencing TC Rainfall Environmental Steering in Northern Hemisphere • Very slow moving TCs and symmetrical TCs produce the most rainfall near the center • Maximum rainfall at night (especially when over land) • Weak steering flow Even rainfall distribution = • TCs that move into a break in the Rain over subtropical ridge often produce smaller area most of the rain right of their track TRACK on right side = Higher totals • TCs that recurve due to significant upper troughs in the westerlies often produce most of their rain left of their track • Rainfall may spread well in advance of the TC due to interaction with the upper jet on the leading edge of the trough Factors Influencing TC Rainfall Predecessor Rainfall Events Predecessor Rainfall Event (PRE) Image from Matt Cote and Lance Bosart – SUNY Albany Revised & Updated from Fig. 13, Bosart and Carr (1978) • Moisture transport well ahead of TC itself • Coherent area of rain displaced north of the TC (near a front or over terrain) • Maximum rainfall rates can exceed 200 mm in 24 hr • Occurs for approximately 1 of 3 landfalling TCs in U.S. Factors Influencing TC Rainfall Ida (2009) Disturbance 0015 UTC 0315 UTC 8 Nov 8 Nov 0615 UTC 0915 UTC 8 Nov 8 Nov “Twin” Disturbance or Other Secondary Features Where is Flooding from Tropical Cyclones Likely to Occur? • Areas where the ground is already saturated (low flash flood guidance values) • Valleys/watersheds Costa Rica • Areas of orographic stream flooding enhancement • Areas with poor drainage or prone to runoff Clear Cut Areas in Haiti • Areas with directed drainage that can be overwhelmed TC Rainfall Forecasting Tools NHC Rainfall Product Incorporates microwave (MW) satellite data rainfall rates • NHC uses two different merged satellite rainfall estimation techniques: - NRL-Blend and QMORPH incorporate available MW data and propagate precipitation forward in time via IR - Training on the NRL-Blend technique: http://www.nrlmry.navy.mil/training-bin/training.cgi • As a third product, NHC uses the last applicable GFS forecast - A model forecast has the advantage of dynamics, topography, moisture, etc.